In order to minimize the space required by display devices, research into the development of various flat panel display devices such as LCD display devices, plasma display panels (PDP) and electro-luminescence displays (EL), has been undertaken to displace larger cathode-ray tube displays (CRT) as the most commonly used display devices. Particularly, in the case of LCD display devices, liquid crystal technology has been explored because the optical characteristics of liquid crystal material can be controlled in response to changes in electric fields applied thereto.
At present, the dominant methods for fabricating liquid crystal display devices (LCD) and panels are methods based on amorphous silicon (a-Si) thin film transistor (TFT) technologies. Using these technologies, high quality image displays of substantial size can be fabricated using low temperature processes. As will be understood by those skilled in the art, conventional LCD devices typically include a transparent (e.g., glass) substrate with an array of thin film transistors thereon, pixel electrodes, orthogonal gate and data lines, a color filter substrate and liquid crystal material between the transparent substrate and color filter substrate. The use of a-Si TFT technology typically also requires the use of separate peripheral integrated circuitry to drive the gates and sources (i.e., data inputs) of the TFTs in the array. Therefore, there is typically provided a large number of pads for connecting the gate lines (which are coupled to the gates of the TFTs) and data lines (which are coupled to the sources of the TFTs) to the peripheral drive circuitry.
FIG. 1 is a diagram illustrating a schematic layout of a conventional LCD display device. Here, plurality of gate lines 3 and plurality of data lines 7 are arranged in a substrate 1 in a matrix format. A plurality of gate pads 5 and a plurality of data pads 9 are also provided at ends of the gate lines 5 and the data lines 7, respectively. A portion of the device enclosed by one gate line 3 and one data line 7 typically forms a pixel 11. In addition, FIG. 2 is a flowchart illustrating five steps of a conventional method of forming a TFT-LCD display device, and FIGS. 3-5 are sectional views illustrating a TFT-LCD manufactured by the conventional method of FIG. 2.
A conventional method for manufacturing a TFT-LCD display device will now be described with reference to FIGS. 2-5. First, a first metal layer, having a stacked structure including chromium (Cr) and an aluminum (Al) alloy, is formed on a transparent glass substrate 100 to a predetermined thickness. Then, the first metal layer is etched by a first photolithography process to form a gate electrode 10 and a gate line 10' on a TFT portion and gate pad portion of the substrate 100 (step 101). Then, a layer (e.g., nitride layer) is deposited on the entire surface of the substrate having the gate electrode 10 and the gate line 10' thereon to form a gate insulation layer 12. An amorphous silicon layer and an impurity-doped amorphous silicon layer are then sequentially deposited on the gate insulation layer 12 to form an amorphous semiconductor layer. Next, the amorphous semiconductor layer is patterned by a second photolithography process, resulting in a semiconductor layer pattern 14 on the TFT portion of the substrate 100 (step 102).
Then, a second metal layer such as Cr is deposited on the entire surface of the insulation layer 12 and on the amorphous semiconductor layer pattern 14 to a predetermined thickness. The second metal layer is then patterned by a third photolithography process to form a data line 16a and a source/drain electrode 16b on the TFT portion of the substrate, a gate pad 16c on the gate pad portion of the substrate, and a data pad 16d on a data pad portion of the substrate (step 103), as illustrated by FIGS. 3-5, respectively.
A passivation layer 18 is then formed on the entire surface of the above structure to a predetermined thickness. The passivation layer 18 is then patterned to expose parts of the drain electrode 16b, the gate line 10' and data pad 16d using a fourth photolithography process (step 104). After forming an indium-tin-oxide (ITO) layer as a transparent conductive layer on the entire surface of the structure having the passivation layer pattern 18 thereon, the ITO layer is patterned by a fifth photolithography process to form a pixel electrode 20 (step 105).
Unfortunately, the use of chromium (Cr) as the second metal layer may not be preferred as a data line material because it typically has a relatively high resistivity. This relatively high resistivity can lead to an increased RC delay associated with the data line and can reduce the maximum viewing angle of the display. The use of chromium as the second metal layer may also be limited by the frequency of formation of metal line discontinuities during processing which can reduce device yield. Also, the use of aluminum (Al) or an alloy thereof may not be preferred because contact formation between aluminum based alloys and indium-tin-oxide (ITO) layers typically results in the formation of aluminum oxide clusters. These oxide clusters typically act as electrical insulators and increase contact resistance. As will be understood by those skilled in the art, these insulating clusters are typically formed when current passes through the aluminum/ITO contacts and causes aluminum atoms to migrate into the ITO. This parasitic phenomenon is typically referred to as "metal migration".
Thus, notwithstanding the above described method of forming TFT-LCD devices, there continues to be a need for improved methods of forming TFT-LCD display devices.